Make sure that KDE is not configured for multiple desktops.
To configure that, open the KDE Control Center, select
Desktop > Multiple desktops, and set the number
of desktops to 1.

Read the guided tour of xmonad,
so that you'll know what to do once xmonad starts up.
(Ignore the "Starting xmonad" section, though.
These instructions replace that for KDE.)
The xmonad man page
has a nice summary of the default key bindings for xmonad.

5 Sample xmonad configuration for KDE

As usual, place xmonad configuration in ~/.xmonad/xmonad.hs.

The following sample configuration sets up xmonad to cooperate
with the KDE desktop and panel; for more details about
how this works, see the
Gnome page.

This configuration also does the following:

uses the Windows key instead of the Alt key as "mod" for xmonad (freeing up Alt for common emacs-style key bindings in applications)

causes certain applications to launch as floating windows

automatically sends certain applications to a specific desktop when they launch.

Enter the command xprop | grep WM_CLASS in a terminal window on the same desktop.

Click on the application window.

Read the class name in the terminal window. The class name is the second of the two quoted strings displayed, usually capitalized.

Thanks to everyone on #xmonad,
especially sjanssen,
for all the help in putting together this sample xmonad configuration
for KDE.

6 Make xmonad your window manager in KDE

Create the directory ~/.kde/env if it does not
already exist. Create a file there called set_window_manager.sh
containing only the following line of text:

KDEWM=/path/to/xmonad

where "/path/to/xmonad" is the path to the xmonad
binary on your system. For example, on Debian systems this
is /usr/bin/xmonad. If you compiled xmonad
from source, it may be something like /home/$USER/bin/xmonad.

7 Restart your KDE session

Now end your current KDE session and start a new one.
Welcome to xmonad with KDE!

8 Tips and issues

As in Gnome, you currently cannot switch the focus to a window by clicking on it in the task bar. Use xmonad keys, or use the mouse on the window itself.

Also as in Gnome, it is very important not to use the xmonad mod-shift-q key to exit your session. Use the KDE menu or panel applet.

To do: In the sample configuration, show how to bindmod-shift-Qto the commanddcop kdesktop default logoutto end the KDE session via a dialog, or to the commanddcop ksmserver default logout 0 -1 -1to end the KDE session directly. Then remove the above bullet.